I remembered that I had written an enthusiastic post over on virtualpetlist a few months ago in the thread "What do pet sites and sim games mean to you?" Here's a copy:
When I was a kid, what I really wanted to be when I grew up was a genetic engineer, doing stuff like reviving dinosaurs from DNA trapped in amber and genetically engineering pegasi and that sort of thing. I also wanted to be a survivalist, running my own farm and ranch, and cooking the crops and meat I had produced. Even recently I briefly had an ambition to breed some new flower varieties. But these kind of things are so ridiculously difficult to do in reality that they are either simply impossible or too grueling to be fun or way more expensive than more normal ways of earning a living and keeping oneself fed. Games where I can breed animals or grow plants help me get back to that childlike enthusiasm where I wanted to add something new and cool to the world, or at least master its mechanics and wield them like an artist, back before I realized how much the world sucks and that everything easy enough to plausibly do has already been done.
Similarly I enjoy games where I can look like whatever I want (through an avatar), dress "myself" up in stylish clothes, and/or get rich for being the best at some profession, and given respect and fame for my abilities. All things I wish I could have in real life but most likely never will.
Edit: someone linked me to Rush's Closer To The Heart today and I laughed at how succinctly it expresses the enjoyment of sim game play